The present invention relates to an overload protection apparatus for protecting a motor and a driving mechanism driven by the motor from an overload, and concerns a speed reducer provided with such an apparatus.
FIG. 1 is a circuit diagram that shows a feeder circuit for a three-phase motor and a conventional overload protection apparatus for protecting a driving mechanism driven by the three-phase motor from an overload, and respective lines from a three-phase power source of R, S and T phases, not shown, are connected to three lines of U, V and W phases respectively connected to three terminals of a motor M with a molded case circuit breaker (CB) 91 interpolated in between. The respective lines of U, V and W phases are connected to the three-phase motor M for driving, for example, a conveyor belt, with a electromagnetic contactor (MC) 92 interpolated in between. The lines of U and W phases, which connect the electromagnetic contactor 92 and the three-phase motor M, are provided with thermal relays (OCR) 93, and the line of W phase is further provided with a current transformer 94. The output of the current transformer 94 is inputted to the input terminals k and l of an overload protection apparatus 90. The overload protection apparatus 90 is interpolated between the U-phase line and the W-phase line, and between the b terminal of the overload protection apparatus 90 and the W-phase line are interpolated a push button switch for stopping 95, a parallel circuit between a push button switch for starting 96 and an electromagnetic contactor 92, and a series circuit consisting of the electromagnetic coil of the electromagnetic contactor 92 and a normally-closed contact of a thermal relay 97.
The following description will discuss the operation of such an overload protection apparatus 90. When the push button switch for starting 96 is closed with the molded case circuit breaker 91 being closed, the electromagnetic coil of the electromagnetic contactor 92 is excited, the electromagnetic contactor 92 is closed, and three-phase currents are allowed to flow through the three-phase motor M so that the motor M is started. Here, the current transformer 94 detects the motor currents, and the resulting output is inputted to the overload protection apparatus 90, with the result that the overload protection apparatus 90 obtains an output voltage V that is proportional to the motor driving current I. Then, the overload protection apparatus 90 compares the levels of the output voltage V and a starting detection reference voltage E1, and when V&gt;E1, detects the starting of the three-phase motor M.
After a predetermined time has elapsed from the detection of the starting, the output voltage V obtained by the overload protection apparatus 90 and an overload detection reference voltage E2 are compared in their levels, and when V&gt;E2, time counting is started, and after a predetermined time has been counted, the electromagnetic coil is demagnetized, with the result that the electromagnetic contactor 92 is opened, and the motor driving current I is cut off so that the motor M is stopped; thus, the motor and the driving mechanism are protected from the overload.
However, in the conventional overload protection apparatus of this type, the voltage V that is proportional to the motor driving current I is detected, and this is compared with the overload detection reference voltage E2 that has been preliminarily set, and if the voltage V exceeds the reference voltage E2, it is judged as an overload so that a trip signal is outputted; thus, the electromagnetic contactor 92 for opening and closing the power supply line to the motor M is allowed to open by the trip signal so that the motor driving current is cut off.
However, the following problems arise in the conventional arrangement as described above.
(1) Since the motor driving current is cut off by disconnecting the power supply line by using the electromagnetic contactor 92, a complex wiring job is required upon installing the overload protection apparatus 90. PA0 (2) It is difficult to install the overload protection apparatus in a motor driving system without the electromagnetic contactor 92, such as a motor driving system using a knife switch or a push button switch. PA0 (3) In the driving mechanism driven by a motor, in the case of a low ambient temperature, an increased motor output torque is required for driving due to high viscosity in the grease, etc., and an increased motor driving current is consequently required. Therefore, in the case of a lower ambient temperature as compared with the temperature at the time of the setting of the overload detection reference value of the overload protection apparatus, there is a possibility that the overload protection apparatus might trip unnecessarily.